1. Field of the Invention
This invention relates in general to a thread mill for forming an external or internal thread in an outer or inner circumferential surface of a workpiece, and more particularly to such a thread mill that is capable of forming the thread without undesirable debris or burrs being left on the threaded circumferential surface.
2. Discussion of the Related Art
As a kind of tool for forming an internal thread in an inner circumferential surface of a hole previously formed in a workpiece, there is known a thread mill including a cylindrical main body having annular protrusions formed on its outer circumferential surface. Each of the annular protrusions has a profile of each ridge of the internal thread that is to be formed, and extends in a circumferential direction of the cylindrical main body without extending in an axial direction of the cylindrical main body for providing no lead. The cylindrical main body has flutes formed in the circumferential surface, so that each of the annular protrusion is divided by the flutes into a plurality of portions. The thread mill is attached to a machining center or other machine tool having numerically controlled three or more axes, and is moved along a helical interpolation path within the formed hole while being rotated about its axis, for machining or forming the internal thread in the circumferential surface of the formed hole, with cutting edges each provided by a rear-side one of widthwise opposite edges of a corresponding one of the flutes. An example of such a thread mill is disclosed in JP-S63-200916A (publication of unexamined Japanese Patent Application laid open in 1988). This thread mill disclosed in the Japanese publication has, in addition to the cutting edges for forming an internal thread, cutting edges provided in an axially distal end portion of a fluted main body and cutting edges provided in an axially proximal end portion of the fluted main body. The cutting edges provided in the axially distal end portion serve to form a hole having an inner circumferential surface in which the internal thread is to be formed, while the cutting edges provided in the axially proximal end portion serve to form a beveled surface in an opening edge of the hole.
In a conventional thread mill such as the thread mill disclosed in the above identified Japanese publication, each of the flutes or cutting edges twists in the same direction as a rotating direction of the tool. That is, the conventional thread mill is a right-hand cutting tool in which each flute or cutting edge twists in a clockwise direction as seen from its shank, so that chips are evacuated toward the shank when an internal thread is cut by the thread mill rotated about its axis in the clockwise direction. However, in such a conventional thread mill, a portion of each cutting edge adjacent to a rear-side (i.e., shank-side) one of flanks (hereinafter simply referred to as “rear flank”) of each annular protrusion provides a negative rake rather than a positive rake, namely, provides a rake angle that is obtuse rather than acute. Therefore, the portion of each cutting edge adjacent to the rear flank of each annular protrusion cannot exhibit a satisfactory cutting performance, resulting in burrs left in an entrance opening of the hole or a chamfered portion of the formed internal thread, or burrs partially removed from the chamfered portion of the internal thread. The burrs left in the chamfered portion of the internal thread is problematic, particularly, where the thread mill has annular protrusions each having a profile configured to cut each thread ridge including its crest, namely, where the burrs are unlikely to be left in each crest in a complete thread portion of the internal thread.
FIGS. 4A and 4B are views showing, by way of example, burrs left in an internal thread, which was formed by using a thread mill which has a multiplicity of annular protrusions having a major diameter of 9.5 mm and a pitch of 1.25 mm (φ9.5 mm×P 1.25), under a cutting condition as specified below. This thread mill has right-hand spiral flutes each twists by a helix angle of 15° in a clockwise direction, which corresponds to a rotating direction of the tool as seen from its shank. FIG. 4A shows a burr left in an entrance opening of a hole or a chamfered portion of the internal thread, while FIG. 4B shows a moustache-like burr partially removed from the chamfered portion of the internal thread. Such a burr remaining in the chamfered portion of the internal thread could impede engagement of the internal thread with an external thread.
Cutting Condition
    Material of workpiece: SS400    Diameter of prepared hole: φ10.8 mm    Amount of chamfering opening edge of prepared hole: C 1.5    Size of internal thread: M12×P 1.25    Cutting fluid: Water soluble coolant    Used machine: Horizontal machining center